Assemble: allow to assemble spares on their own

Assemble: allow to assemble spares on their own

If we find spares but no members of given array
we create container with just spares.

This allows auto assemble to pick up all lose imsm spares when there
is no config file.
When there is a valid config file and any array is assembled from it
we don't try auto assembly so we will not assemble spares that don't
match any array.
To remedy this we must add
ARRAY metadata=imsm UUID=00000000:00000000:00000000:00000000
to config file.
This container will include all remaining spares.

Signed-off-by: Anna Czarnowska <anna.czarnowska@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>